pgsql: Code cleanup for assign_transaction_read_only.

Code cleanup for assign_transaction_read_only.

As in commit fb4c5d2798730f60b102d775f22fb53c26a6445d on 2011-01-21,
this avoids spurious debug messages and allows idempotent changes at
any time.  Along the way, make assign_XactIsoLevel allow idempotent
changes even when not within a subtransaction, to be consistent with
the new coding of assign_transaction_read_only and because there's
no compelling reason to do otherwise.

Kevin Grittner, with some adjustments.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=6f59777c65d557485e933a383ebc4c3fdfc1a2b7

Modified Files
--------------
src/backend/commands/variable.c            |   53 +++++++++++++++++++-
src/backend/utils/misc/guc.c               |   29 -----------
src/include/commands/variable.h            |    2 +
src/test/regress/expected/transactions.out |   75 ++++++++++++++++++++++++++++
src/test/regress/sql/transactions.sql      |   42 +++++++++++++++
5 files changed, 170 insertions(+), 31 deletions(-)